The present invention relates to pumps. More specifically, the present invention relates to rotary gear pumps and, still more specifically, the present invention relates to coatings for gears used in rotary gear pumps that provide wear resistance and lubrication properties to the gears.
Gear pumps typically utilize two gears. One gear is referred to as the driver and is connected to the input power shaft. The other gear is referred to as an idler gear and rotates with the driver gear. In most gear pumps, the two gears mesh, i.e. then engage one another. Accordingly, lubrication and wear can pose serious problems that affect the performance of the pump and the frequency in which the gears must be changed.
Further, gear pumps are typically used to pump fire fighting foam. These pumps must operate at pressures ranging from 100 psig to 400 psig. While these pressures are not ordinarily problematic with hydraulic gear pumps that pump oil, hydraulic fluid or some other lubricating fluid, these pressures are particularly problematic with pumps that pump fire fighting foam chemicals because the foam chemicals are not good lubricants. Some fire fighting foams are fabricated from animal matter and can have abrasive bits of bone and other impurities that can increase the wear of the gears. Other synthetic foams can be very thin and therefore unable to support a hydrodynamic film that acts as a lubricant on the exterior of the gear. While a foam material can appear thick when contained in a vessel, foams can be extremely watery and thin as the foam is sheared inside a pump chamber under the action of two rotating gears. Some foam chemicals are thixotropic, and become extremely low viscosity when sheared while maintaining high viscosity while at rest.
Still further, fire fighting foams and chemicals can be corrosive. Aqueous solutions will rust ordinary carbon steels and therefore the use of bronze or 300 series stainless steels are required to combat corrosion. However, these materials do not wear very well and have a tendency to gall. Accordingly, bronze and stainless steel gears wear our prematurely and are not preferred materials for use at high pressures.
One solution to the aforenoted problems has been the development of pumps with external gear timing. Specifically, the rotation of the two gears is timed in such a way so as to actually prevent the gear teeth from engaging one another and therefore imposing wear upon one another. However, the external timing gears must be disposed in oil and sealed from the pump chamber containing the pump gears. Because of the high pressures involved in pumping fire fighting chemicals and foam, additional shaft seals, an oil bath housing and other miscellaneous parts are required to isolate the timing gears from the high pressure environment of the pump chamber and pump gears. As a result, gear pumps that employ external gear timing are complex and costly. Further, the numerous potential leak points inherent in these designs create additional maintenance problems.
As a result, there is a need for an improved rotary gear pump having improved gears that wear longer and resist galling to a greater extent than currently available rotary gears. Additionally, there is a need for an improved rotary gear pump that can operate dependably at high pressures for extended periods of time without the need for resorting to an external gear timing design.